The CCR5 co-receptor is used by all strains and clades of HIV-1 during person-to-person transmission. This project seeks to better understand the interactions that occur between gpl2O and CCR5 during the process of viral entry, in order to gain insights into the problems that might be encountered in developing a vaccine that induces broadly cross-reactive neutralizing antibody responses. The applicants hypothesize that there is plasticity to the gpl20/CCR5 interaction, and that M-tropic strains of HIV-1 can use multiple different extracellular domains on CCR5 during the entry process. The overall goal of this proposal is to use a panel of chimeric chemokine receptor molecules, HIV-1 strains, and monoclonal antibodies to various extracellular domains of CCR5 to determine the plasticity of the native gpl2O/CCR5 interaction, identify regions of CCR5 that facilitate HIV-1 gpl2O binding or ligand binding, and determine if blocking the availability of any single extracellular domain on CCR5 is adequate to block all CCR5-dependent HIV-1 entry. To accomplish these goals the following specific aims are projected: 1) to determine whether the CCR5 interactive sites on HIV-1 envelope glycoproteins are conserved between isolates and across clades, and whether all envelopes can be inhibited from using CCR5 as an entry co-receptor through blocking a single extracellular domain on CCR5; and 2) to determine whether envelopes from different isolates or different clades will adapt to use other regions of CCR5 for viral entry, or whether they will alter their affinity for CCR5, in order to overcome a block to entry imposed by a specific blocker of the primary gpl20/CCR5 interactive site.